It is well recognized by persons skilled in the art of recovering oil or petroleum from subterranean deposits thereof that only a small fraction of viscous petroleum may be recovered from subterranean formations by conventional primary and secondary means, and some method such as a thermal recovery process or other treatment must be applied to the formation to reduce the viscosity of the petroleum to a level at which it will flow readily to wells from which it can be recovered to the surface of the earth. Steam and/or hot water flooding are commonly used for this purpose, and have been very successful in some formations for stimulating recovery of viscous petroleum which is otherwise essentially unrecoverable.
Problems are encountered in applying steam flooding to subterranean formations containing very viscous petroleum, which are especially severe if the vertical thickness of the formation is quite high. Because steam is much less viscous than the viscous petroleum present in the formation, and further because the density of vapor phase steam is significantly less than the density of petroleum, there is a strong tendency for steam injection even near the bottom of the oil-saturated portion of the formation, to channel into the top of the formation, referred to as steam override. Once steam override has begun, continued injection of steam into the formation will accomplish very little additional petroleum recovery. Steam breakthrough at the production well completes a high permeability path between the injection well and the production well across the top of the formation, and steam injected thereafter will continue to channel through the steam-swept channel at the top of the formation and will displace very little additional viscous petroleum from the portion of the formation below the channel. This problem is so severe that vertical sweep efficiencies as low as 10-15 percent are not uncommon. The poor mobility ratio between steam and viscous petroleum also results in relatively poor horizontal conformance, and so the total volumetric sweep efficiency of a steam flood is poor.
In view of the foregoing discussion, it can be appreciated that there is a significant need for improving the vertical and horizontal sweep efficiency of steam flooding and other thermal oil recovery methods.
While techniques have been proposed in the art for treating the injection well to cure an adverse permeability distribution problem, these treatments only affect the portion of the formation immediately adjacent to the wellbore. While the permeability of the steam zone in the portion of the formation immediately adjacent to an injection well may be treated to reduce its permeability, the effect is limited to the near wellbore zone and steam quickly channels back to the original swept portion of the formation, once the fluid has traveled a short distance away from the injection well where the treatment was applied.
In view of the foregoing discussion, it can be appreciated that there is a need for a method for treating the formation so as to effect reduction in the permeability of the steam-swept channels at significant distances between the injection well and the production well.